Stationary radiographic imaging equipment are employed in medical facilities (e.g., in a radiological department) to capture medical x-ray images on an x-ray detector. Mobile carts can also be used and include an x-ray source used to capture (e.g., digital) x-ray images on x-ray detectors. Such medical x-ray images can be captured using various techniques such as computed radiography (CR) and digital radiography (DR) in radiographic detectors. A related art digital radiography (DR) imaging panel acquires image data from a scintillating medium using an array of individual sensors, arranged in a row-by-column matrix, in which each sensor provides a single pixel of image data. Each pixel generally includes a photosensor and a switching element that can be arranged in a co-planar or a vertically integrated manner, as is generally known in the art. In these imaging devices, hydrogenated amorphous silicon (a-Si:H) is commonly used to form the photodiode and the thin-film transistor switch needed for each pixel. In one known imaging arrangement, a frontplane has an array of photosensitive elements and a backplane has an array of thin-film transistor (TFT) switches connected thereto.
FIG. 1 is a perspective view of a digital radiographic (DR) imaging system 10 that includes a generally planar DR detector 40 (shown without a housing for clarity of description), an x-ray source 14 configured to generate radiographic energy (x-ray radiation), and a digital monitor 26 configured to display images captured by the DR detector 40, according to one embodiment. The DR detector 40 may include a two dimensional array 12 of detector cells 22 (photosensors), arranged in electronically addressable rows and columns. The DR detector 40 may be positioned to receive x-rays 16 passing through a subject 20 during a radiographic energy exposure, or radiographic energy pulse, emitted by the x-ray source 14. As shown in FIG. 1, the radiographic imaging system 10 may use an x-ray source 14 that emits collimated x-rays 16, e.g. an x-ray beam, selectively aimed at and passing through a preselected region 18 of the subject 20. The x-ray beam 16 may be attenuated by varying degrees along its plurality of rays according to the internal structure of the subject 20, which attenuated rays are detected by the array 12 of photosensitive detector cells 22. The planar DR detector 40 is positioned, as much as possible, in a perpendicular relation to a substantially central ray 17 of the plurality of rays 16 emitted by the x-ray source 14. The array 12 of individual photosensitive cells (pixels) 22 may be electronically addressed (scanned) by their position according to column and row. As used herein, the terms “column” and “row” refer to the vertical and horizontal arrangement of the photosensor cells 22 and, for clarity of description, it will be assumed that the rows extend horizontally and the columns extend vertically. However, the orientation of the columns and rows is arbitrary and does not limit the scope of any embodiments disclosed herein. Furthermore, the term “subject” may be illustrated as a human patient in the description of FIG. 1, however, a subject of a DR imaging system, as the term is used herein, may be a human, an animal, an inanimate object, or a portion thereof.
In one exemplary embodiment, the rows of photosensitive cells 22 may be scanned one or more at a time by electronic scanning circuit 28 so that the exposure data from the array 12 may be transmitted to electronic read-out circuit 30. Each photosensitive cell 22 may independently store a charge proportional to an intensity, or energy level, of the attenuated radiographic radiation, or x-rays, received and absorbed in the cell. Thus, each photosensitive cell, when read-out, provides information defining a pixel of a radiographic image 24, e.g. a brightness level or an amount of energy absorbed by the pixel, that may be digitally decoded by image processing electronics 34 and transmitted to be displayed by the digital monitor 26 for viewing by a user. An electronic bias circuit 32 is electrically connected to the two-dimensional detector array 12 to provide a bias voltage to each of the photosensitive cells 22.
Each of the bias circuit 32, the scanning circuit 28, and the read-out circuit 30, may communicate with an acquisition control and image processing unit 34 over a connected cable (wired) 33, or the DR detector may be equipped with a wireless transmitter to transmit radiographic image data wirelessly 35 to the acquisition control and image processing unit 34. The acquisition control and image processing unit 34 may include a processor and electronic memory (not shown) to control operations of the DR detector 40 as described herein, including control of circuits 28, 30, and 32, for example, by use of programmed instructions. The acquisition control and image processing unit 34 may also be used to control activation of the x-ray source 14 during a radiographic exposure, controlling an x-ray tube electric current magnitude, and thus the fluence of x-rays in x-ray beam 16, and/or the x-ray tube voltage, and thus the energy level of the x-rays in x-ray beam 16.
The acquisition control and image processing unit 34 may transmit image (pixel) data to the monitor 26, based on the radiographic exposure data received from the array 12 of photosensitive cells 22. Alternatively, acquisition control and image processing unit 34 can process the image data and store it, or it may store raw unprocessed image data, in local or remotely accessible memory.
With regard to a direct detection embodiment of DR detector 40, the photosensitive cells 22 may each include a sensing element sensitive to x-rays, i.e. it absorbs x-rays and generates an amount of charge carriers in proportion to a magnitude of the absorbed x-ray energy. A switching element may be configured to be selectively activated to read out the charge level of a corresponding x-ray sensing element. With regard to an indirect detection embodiment of DR detector 40, photosensitive cells 22 may each include a sensing element sensitive to light rays in the visible spectrum, i.e. it absorbs light rays and generates an amount of charge carriers in proportion to a magnitude of the absorbed light energy, and a switching element that is selectively activated to read the charge level of the corresponding sensing element. A scintillator, or wavelength converter, is disposed over the light sensitive sensing elements to convert incident x-ray radiographic energy to visible light energy.
Examples of sensing elements used in sensing array 12 include various types of photoelectric conversion devices (e.g., photosensors) such as photodiodes (P-N or PIN diodes), photo-capacitors (MIS), photo-transistors or photoconductors. Examples of switching elements used for signal read-out include MOS transistors, bipolar transistors and other p-n junction components.
The standard exposure process for a DR detector system would comprise commanding the detector to enter a charge integration mode in which the photosensors are ready to receive a signal, then commanding the generator to prepare to fire for a known length of time and at a known X-ray flux, and, finally, under the command of the operator, generating an X-ray pulse. At the end of the X-ray exposure time, the detector and/or the acquisition control and image processing unit commands the detector to read out the image, using row by column readout electronics, from the photodiodes to electronic memory in the detector 40 or in the processing unit 34. The detector and/or the acquisition control and image processing unit may also command the detector to obtain one or more dark reference frames prior to or following the X-ray exposure for dark correction of the exposed image.
In many medical facilities, especially those with pre-existing X-ray generators, there is no pre-existing communication link between generator and detector. In many cases, these generators were intended for use with X-ray film or computed radiography phosphor plates. For these cases, or for convenience and reduction in wiring, it is desirable for the detector to be able to independently sense the start of X-ray exposure and enter a charge integration state as well as sense the end of exposure and enter a readout state.
A diagram of an exemplary pixel in a prior-art pixel passive array is shown in FIG. 2. The pixel 200 comprises a photo-diode 202 and a row-select thin-film transistor (TFT) 204. The anode 201 of the photodiode 202 is connected to a bias line 206 which supplies a constant bias voltage to the photodiode and the cathode 203 of the photodiode is connected to the drain of the row-select TFT 204. The gate of the row-select TFT is controlled by a gate line 208. The source of the TFT is connected to a data line 210.
A top view diagram of the layered layout of a pixel 300 is shown in FIG. 3 and a cross-section along A-A″ of the pixel's layout 400 is shown in FIG. 4. The gate line metal 308 is deposited on the substrate 402 and patterned to form the gate line 308. The TFT 404 is formed by depositing and patterning in succession the gate dielectric 406, intrinsic and N+ amorphous silicon for the TFT channel 408, the source-drain metal to form TFT drain 410 and TFT source and data line 310, and a TFT passivation dielectric 414. The photodiode 302 is then formed by successive deposition and patterning of a cathode 203, followed by N+, intrinsic and P+ amorphous silicon layers, a transparent anode 201, a photodiode passivation dielectric 416, and a metal bias line 306.
A schematic of a photodiode array is shown in FIG. 5. In this prior-art array 500 the photo-sensing element is a PIN photodiode 502 and the switching device is a TFT 204 with source, gate and drain, as described above with reference to FIG. 2, although other photo-sensing elements and alternative switching elements may be employed. The gate lines 208 are oriented along the row direction and typically connects the gates of all the pixels in a row to a row address circuit 504. The row address circuit, which is positioned peripheral to the array, sequentially addresses each row, momentarily switching the TFT in the pixels along that row from an insulating (off) into a conducting (on) state. The source of the TFT is connected to a data line 210, which is oriented in the column direction of the array and is typically connected to all pixels in that column. The rows and columns may be referred to herein as comprising horizontal and vertical directions, respectively, which are arbitrary reference directions that are helpful for discussion purposes. Each data line is connected to a signal sensing circuit 506 peripheral to the array. In the circuit of FIG. 5, the signal sensing circuit comprises a charge amplifier 508 including an operational amplifier 510, a feedback capacitor 512, a reference voltage supply 514, and a feedback reset switch 516. The charge amplifier 508 senses the amount of charge required to reset the data line to the reference voltage by measuring the charge on the feedback amplifier 508.
In typical operation, the charge amplifiers 508 reset the cathodes of all photodiodes in the array to VREF by sequentially addressing each row of pixels. The timing sequence 600 is briefly described with reference to FIG. 6. The charge amplifier is illustrated in more detail in FIG. 7 and consists of a differential operational amplifier 710 with reference voltage 514 applied to the negative terminal 701 and signal input to the positive terminal 702. The input can be isolated from the charge amplifier by an isolation switch 704. A reset sample switch RSS 706, reset sample capacitor 708, a signal sample switch SSS 710, and signal sample capacitor 712 are illustrated connected to the output of operational amplifier 710. An integrator reset switch IRS 516 is provided between the input and output of the amplifier 710.
With respect to FIG. 6, at the start of a line time TL, with the isolation switch 704 closed, the reset and signal sample capacitors 708, 712, are reset by closing the reset sample switch (RSS) 706, the signal sample switch (SSS) 710 and the integrator reset switch (IRS) 516. The reset signal charge level is then sampled by opening IRS 516 and SSS 710 while closing RSS 706. The analog signal processing following the charge amplifier can then sample the reset charge level at the output 718 of the charge amplifier 508. At the end of the reset sample, RSS 706 is opened and SSS 710 is closed. The gate line of the addressed row is then switched from VOFF to VON, transferring charge from the cathode of the diode in the addressed row to the signal sample capacitor 712. After a gate line on-time TGL-ON 602, the gate line is switched back from VON to VOFF. After the signal level has settled on the output 718 of the charge amplifier 508, the analog charge level on the output 718 of the charge amplifier 508 can be sampled by the analog signal processing, and the SSS 710 closes. The voltage difference between the signal and reset levels represents the signal charge divided by the feedback capacitance.
The dataline is reset to VREF by closing the feedback switch 516 of the charge amplifier 508. The timing is illustrated in FIG. 6. With the charge amplifier feedback switch 516 open, the row select circuit 504 addresses a gate line for switching a single gate line from VOFF to VON for a time on-time TGL-on 602. While the TFT gate is on, the charge from the photodiode is transferred to the dataline. The charge amplifier 508 transfers that charge to the feedback capacitor 512 of the charge amplifier 508 for each dataline. The photodiode voltage is thus reset to VBIAS−VREF (515, 514). The charge amplifier 508 completes the filtering and integration of the charge signal, then connects the output 718 of the charge amplifier 508 to the analog-to-digital converter. The feedback reset switch 516 is then closed, resetting the capacitor 512 and continuing to hold the data line at VREF.
Prior to exposure by X-rays, the array is placed in integrate mode in which the gate lines of all the photodiodes are held at VOFF to isolate the diodes from the data line. In the presence of X-ray exposure, the photo-charge is stored on the photodiodes in addition to charge from the dark current of the photodiodes. Following exposure the charge in the array may be read out by sequentially addressing each of the rows, transferring the charge in the photodiodes in that row to the respective data lines, and sensing the charge in the charge amplifier 508 connected to each of the data lines. In typical operation, one or more unexposed frames would be captured either before and/or after exposure and digitally subtracted from the exposed frame.
The image sensor of FIG. 5 does not include circuitry to independently sense the start and end of X-ray exposure. For this sensor, the timing of the imaging array and the timing of the generator must be synchronized externally.
Several modifications to the image sensor design and operation have been proposed for systems in which the generator and the detector cannot be synchronized externally. These systems require X-ray beam sensing to determine the start of exposure so that the array may be placed into integrate mode. Examples of beam-sensing circuits include independent X-ray exposure sensing units integrated into the detector cassette below the detector, a detector with a circuit to sense a change in current in the bias supply for the photodiode bias voltage, sensors incorporating a sparse matrix of photodiodes inside the imaging array which are connected to a current-sensing circuit rather than the array readout electronics, etc.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.